The present invention relates to radio communication networks and more precisely to a method for multiplexing two data flows on one radio communication channel.
Transmitters of a radio communication networks usually have to transmit several data flows simultaneously. These data flows may consist in several types of traffic data having several priorities or in signaling messages. Schematically, the transmitter may be represented as supporting as many queues as data flows to transmit simultaneously.
In some radio communication systems the data flows can be transmitted in parallel on independent communication channels. However, in some other radio communication networks, the different data flows have to share the same radio communication channel or at least be processed sequentially at the transmitter so that a completely parallel processing of the different data flow is impossible.
With regard to this matter of facts, a policy determining how the different data flows have to be multiplexed on the single radio communication channel has to be defined at the transmitter. Several usual policies can be envisaged. Usually, each data flow is divided in a succession of data packets, the multiplexing policy being applied at the level of data packets.
One very simple policy named round robin consists in checking each data flow one after the other, in a predefined order. If data packets are available for transmission, one data packet is transmitted on the radio communication channel in the corresponding time slot, if not the next data flow is checked.
Another policy consists in attributing different priorities to the different data flows, a data flow with a higher priority being allowed to transmit data packets with a higher priority than a data flow with a lower priority.
Usually, the data flows with highest priority correspond to the traffic data requiring real time constraints as voice traffic or video. The data flows with lower priority corresponding to non real time traffic data.
Signaling messages, especially signaling messages required for the maintenance of the radio link (e.g. handover messages) occurring during the traffic data transmission phase should be granted the highest priority on the radio communication channel. For example, a handover procedure should be granted a higher priority than the traffic data (even real time traffic data) since the latter will no more be correctly received so long as the handover has not been performed.
However, in recent radio communication systems like GSM/EDGE or UMTS, some signaling messages cannot be contained in one single data packet and have to be segmented in several signaling data packets. The transmission of the complete signaling message (i.e. N signaling data packets) with the highest priority results in that the transmission of data packets belonging to other data flows with lower priority is stopped until the signaling message has been correctly transmitted. If the other data flows are carrying real-time data, the long interruption of transmission for this data flow results in either irrecoverable delay or long muting in the real-time data packet transmission.
This problem is automatically solved in GERAN A/Gb mode using LAPDm as its layer 2 communication protocol for the signaling plane. Indeed, this communication protocol enables only the transmission of a further data packet when an acknowledgement for the preceding packet has been received at the transmitter. This is due to the fact that the transmit window size is limited to one. Then, automatically, the delay between the transmission of two high priority data packets is at least equal to one round trip delay, which is the minimum time required for the transmitter to send a data packet and receive its acknowledgement from the receiving entity. During this time, data packets belonging to traffic data flows with lower priority can be transmitted, reducing to the minimum either the delay in real-time data packets transmission or the pre-emption duration. In this solution, the total time required for transmitting a signaling message segmented in several data packets is very long.
However, when using for the transmission of signaling messages a more efficient layer-2 protocol supporting large sending and receiving windows like the Radio Link Control Protocol as specified in TS 3GPP 44.060 (i.e. not waiting for an acknowledgement of the previous data packet for transmitting the next one), the problem described above remains unsolved.
A particular object of the present invention is to provide a method for multiplexing two data flows guaranteeing that the data flow having the highest priority is transmitted as fast as possible while not delaying or muting too much the transmission of data flow with a lower priority having however real-time constraints.
Another object of the invention is to provide a transmitter of a radio communication network satisfying the two conditions listed above.